System and method for increasing provisionable bandwidth in time-division multiplexed communication links

ABSTRACT

A system for and method of, increasing provisionable bandwidth in a multiplexed communication link and a network management system incorporating the system or the method. In one embodiment, the system includes: (1) a defragmentation calculator that calculates a defragmentation of the communication link, the defragmentation containing at least one granularity, (2) a candidate slot selector, associated with the defragmentation calculator, that finds reduced cost candidate slots for the at least one granularity and (3) a circuit mover, associated with the candidate slot selector, that identifies target slots into which circuits occupying the candidate slots can be moved.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to multiplexedcommunication links and, more specifically, to a system and method forincreasing provisionable bandwidth in time-division multiplexedcommunication links.

BACKGROUND OF THE INVENTION

Synchronous Optical Networking (SONET) and Synchronous, DigitalHierarchy (SDH) are optical network standards. SONET and SDH use TimeDivision Multiplexing (TDM) to subdivide the bandwidth of an opticalchannel into smaller usable fragments called “time slots.” SONET and SDHprovide a hierarchy of bandwidth granularities, specified as “STS-n,”that can be provisioned as end-to-end paths in an optical network.Higher bandwidth granularities are integer multiples of the lowerbandwidth granularities (e.g., STS-1, STS-3c, STS-12c, STS-48c and so onfor SONET).

SONET (see, ANSI standards document T1.105-2001, “Synchronous OpticalNetwork (SONET)—Basic Description including Multiplex Structure, Rates,and Formats”, ITU-T Standards document, incorporated herein byreference) applies to optical networks in North America. SDH (see,G.707—“Network Node Interface for the Synchronous Digital Hierarchy(SDH),” incorporated herein by reference) applies to optical networks inEurope and the rest of the world.

Bandwidth fragmentation is a well-known network engineering issue inservice provider transport networks. Communication links in the networksbecome fragmented as circuits are added and deleted over time, leavingbehind “holes” in the transport pipe. Consequently, network efficiencyis gradually lowered as the free bandwidth is evermore fragmented intosmaller units: new, high-bandwidth circuits cannot be provisioned fromthe fragmented small units.

Fragmentation is a well-studied problem in computing systems going backover 30 years. In the very early computing systems (prior to the use ofmemory paging techniques), the main memory of a computer could becomefragmented as programs got loaded into and erased from the main memory.The most common instance of fragmentation today is disk fragmentation,which causes large files to be “split” into multiple segments whencontiguous free spaces on the disk are too small to hold entire files.In all cases, fragmentation impacts system efficiency, slowing systemperformance and leading to higher operating costs.

While the general problem of fragmentation has been addressed in variouscontexts, the problem of bandwidth fragmentation in optical transportnetworks is novel due to some unique constraints in the transportnetwork. These constraints make prior art defragmentation solutions,such as disk defragmentation solutions, inappropriate.

The SONET and SDH standards stipulate that the bandwidth be provisionedas contiguous time slots on the link. (Due to their similarity from adefragmentation standpoint, only SONET will be referred to hereinafter,with the understanding that SDH is included.) For example, an STS-3c andan STS-12c circuit require three and 12 contiguous time slots of STS-1level granularity respectively, STS-1 being the lowest cross-connectrate.

The provisioning of contiguous time slots is typically referred to as“contiguous concatenation.” As a consequence, fragmentation of a linkcan cause a new demand to be denied, even though sufficient number ofslots exist, because they are noncontiguous.

This “lost” bandwidth can be recovered via a defragmentation operationthat reengineers the circuits to new time slots in order to collate thefree slots. A new standard called “virtual concatenation” (see, G.707,supra.) has been proposed in ITU-T to avoid the fragmentation problem byeliminating the contiguous slot requirement. However, until all edgenetwork elements support the standard, fragmentation will remain aserious issue for SONET networks.

Link defragmentation has come to the fore with the advent of groomingcross-connect switches that enable an incoming signal passing through anode to be switched to any other time slot (Time Slot Interchange).While SONET networks have been traditionally engineered as rings,grooming switches have enabled mesh topologies consisting of theseswitches interconnected by wavelength division multiplexing (WDM) linesystems. Since time slot interchange (TSI) meshes (or rings) enable acircuit to take any slot on each link along the path, each link can bedefragmented independently to locally improve bandwidth efficiency.

The traditional approach in any defragmentation problem is a “push tothe wall,” or PW, operation that moves all existing demands to one endof the container. Referring initially to prior art FIGS. 1A and 1B,shown are two such examples in the context of a SONET link. FIG. 1Ashows the fragmented state of two links 110, 120 connecting a pair ofnodes 130, 140. Thin lines, e.g., lines 150, 160 inside each link 110,120, represent existing circuits. FIG. 1B shows the state of the links110, 120 after a PW operation on each link 110, 120 independently. Suchdefragmentation is referred to as “intra-link defragmentation.”

Defragmentation may also occur across multiple links between the samepair of nodes (so-called “inter-link defragmentation”). Turning now toFIG. 1C, shown is the outcome of such an operation. Besides the normalmotivation of creating contiguous free slots, inter-link defragmentationmay also be performed to free a link 110, 120 so it can be disabled formaintenance.

The success of the defragmentation process is dependent on twometrics—the gain from defragmentation (the recovery of previouslyunprovisional time slots) and the “cost” of achieving the gain. In theSONET context, the cost of defragmentation is the number of circuitsthat move to new slots. Unlike the other instances of defragmentationsuch as memory and disk defragmentation, cost is a critical issue. Thisis because the service provider can defragment a live network and anycircuit move increases the potential for traffic disruption.

Consequently, the goal of any defragmentation algorithm is not only todefragment a link optimally, but to also do it while attempting tominimize the number of circuit moves. Thus, while the PW operationsuffices in other environments as a defragmentation mechanism, it isinappropriate here since it is not always the most cost effective.Accordingly, what is needed in the art is a way to defragmentmultiplexed communication links that takes into account the cost ofdefragmenting.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides a system for and method of, increasingprovisionable bandwidth in a multiplexed communication link and atime-division multiplexer incorporating the system or the method. In oneembodiment, the system includes: (1) a defragmentation calculator (whichmay be an optimum defragmentation calculator) that calculates adefragmentation (perhaps an optimum defragmentation) of thecommunication link, the defragmentation containing at least onegranularity, (2) a candidate slot selector, associated with thedefragmentation calculator, that finds reduced (and perhaps minimum)cost candidate slots for the at least one granularity and (3) a circuitmover, associated with the candidate slot selector, that identifiestarget slots into which circuits occupying the candidate slots can bemoved.

The present invention introduces the broad concept of determining how topack a link so as to free up bandwidth at a reduced cost. Knowledge ofthe optimal packing structure is a necessary step in any defragmentationprocess when cost is a concern. Transitioning a link from its currentstate to a defragmented one with the minimal cost appears to betheoretically hard (intractable and perhaps NP-hard) and may bedifficult to achieve in practical applications. Nonetheless, an optimaldefragmentation algorithm is disclosed herein and guarantees an optimaldefragmentation. An alternative defragmentation algorithm disclosedherein achieves optimal packing in polynomial time, with a very lowdeviation of 10-30% from the minimal cost.

Further disclosed is an algorithm that determines the least costtransition to a defragmented state for any link and use it as abenchmark to compare the performance of the disclosed algorithm. Thisalgorithm is adequate for service provider networks of today.

In one embodiment of the present invention, the candidate slot selectordisqualifies slots containing “nailed-down” circuits from being thecandidate slots. In another embodiment of the present invention, thecandidate slot selector finds only aligned slots as candidate slots forthe at least on granularity.

In yet another embodiment of the present invention, the at least onegranularity is a plurality of granularities related to one another asinteger multiples. Therefore, the system of the present invention can,in various embodiments, defragment a link without disturbing nailed-downcircuits, ensure that candidate slots are aligned and deal withgranularities of different size that are integer multiples of a minimumgranularity.

In one embodiment of the present invention, the multiplexedcommunication link is time-division multiplexed and the slots are timeslots. Thus, a time-division multiplexer constructed in accordance withthe principles of the present invention will further be disclosed.

The foregoing has outlined, rather broadly, preferred and alternativefeatures of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features of the invention will be describedhereinafter that form the subject of the claims of the invention. Thoseskilled in the art should appreciate that they can readily use thedisclosed conception and specific embodiment as a basis for designing ormodifying other structures for carrying out the same purposes of thepresent invention. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention in its broadest form.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIGS. 1A, 1B, and 1C illustrate a prior art PW approach to a linkdefragmentation problem;

FIG. 2 illustrates a schematic of a complete link defragmentationoperation;

FIG. 3 illustrates slots on a single link to demonstrate the completelink defragmentation operation of FIG. 2;

FIG. 4 illustrates a flow diagram of a method of increasingprovisionable bandwidth in a multiplexed communication link carried outaccording to the principles of the present invention;

FIGS. 5A and 5B illustrate pseudocode program listings of portions of analgorithm incorporated in the method of FIG. 4;

FIG. 6 illustrates a block diagram of a system for increasingprovisionable bandwidth in a multiplexed communication link constructedaccording to the principles of the present invention;

FIG. 7 illustrates a block diagram of a network management system (NMS)that incorporates the method of FIG. 4 or the system of FIG. 6; and

FIG. 8 illustrates a pair of graphs showing the performance of theheuristic relative to the optimal algorithm with respect to STS-48 andSTS-192 links.

DETAILED DESCRIPTION

Before describing an algorithm suitable for defragmenting a multiplexedcommunications link, some preferred, but not required, attributes of thedisclosed algorithm will be set forth.

First, the algorithm should preferably be able to handle both intra-linkand inter-link defragmentation requests. While the algorithm shouldpreferably be able to accommodate links of different sizes, they shouldhave similar traffic engineering (TE) and reliability (SRG) constraintsas between the same pair of nodes.

Second, since defragmentation occurs in a live network, the operationshould preferably not disrupt traffic flow. In other words, thealgorithm should preferably not require circuits to be first torn downto achieve packing. Instead, the algorithm's output should enable asequence of “bridge-n-roll” operations to transition circuits seamlesslyto new time slots. Of course, in order for the operation to bedisruption free, the network element (NE) needs to provide bridge-n-rollsupport.

The bridge-n-roll operation works as follows. The NE at one end“bridges,” or replicates, the traffic on the new slot. The NE at theother end now receives the data on both the old and new slots and“merges,” or selects between, the two signals to choose one. To enable acircuit “move,” or a transition to a new slot, the circuit is firstbridged and merged and then the cross-connects on the old slots arereleased leading to a disruption-free transition to the new slot.

Third, the cost of the defragmentation operation is determined by thenumber of circuit moves and should preferably be minimized. This isbecause a failure during the move can have deleterious effects on thenetwork—(1) it can bring down the circuit performing the bridge-n-rolloperation and (2) since a bridge operation temporarily uses up twice thenumber of slots failure during this time can lower the availablebandwidth on the link, in turn impacting the restoration capacity.

Fourth, service providers often have high priority circuits whosedisruption imposes serious penalties and thus, the defragmentationalgorithm should preferably also be able to account for “nailed down”circuits that need to be left untouched in the defragmentation process.

It is this “cost” criterion that in part differentiates the linkfragmentation problem from earlier disk and memory fragmentationsolutions. While it is straightforward to determine the optimal packingstructure given a set of circuits, determining the fewest number ofmoves required to reach the optimal packing structure is an NP-hardproblem. Consequently, the goal of the disclosed algorithm is todefragment optimally while minimizing this moving cost.

FIG. 2 provides a schematic of a complete link defragmentationoperation. On the request of an NMS 210, a link defragmentationalgorithm (LDA) operating in the NMS 210 provides a sequence of circuitsto be moved and their new locations. The NMS 210 then uses this list tomove the circuits, one at a time, to their new slots by a bridge-n-rolloperation as between a master cross-connect 220 and a slavecross-connect 230 (both NEs). Specific portions in the bridge-n-rolloperation will not be detailed here, because they are not bear on anunderstanding of the present invention and are familiar to those skilledin the pertinent art.

A SONET network imposes a set of constraints on the network traffic, andthe illustrated embodiment of the algorithm honors these constraints andoperates under them. The first constraint is a containment property. Thedisclosed algorithm assumes that the bandwidth demands of new circuitsfollow the SONET standard and thus are multiples of each other. In otherwords, the SONET standard calls for STS-<1,3,12,48,192,768>c and so on.Each higher granularity is an integer multiple of each lowergranularity. It will be assumed for simplicity that the lowestgranularity timeslot is STS-1. Therefore, a demand requiring three slotsimplies an STS-3c circuit.

The second constraint is an alignment property. This states that thetime slot for each new circuit demand is aligned to specificpre-determined boundary. For example, a STS-3c circuit can only start onslots 1,4,7,10,13 and so on, a STS-48c circuit can only start on slots1,49,97,145 and so on. Formally, a slot number s is a candidate forstarting slot for circuit of bandwidth d if, and only if, (s mod d)≡1.

The SONET defragmentation problem will now be formally defined. Assuminga link of capacity n (i.e., an STS-n link divided into n STS-1 slots), aset of circuits D={d₁, d₂, . . . d_(k)} and a map Ψ={d_(i)→t_(i)} wheret_(i) is slot number at which demand d_(i) starts. The bandwidth s_(i)of each circuit d_(i) is one of the SONET granularities, i.e., s_(i) ∈{1,3,12,48,192. . . }. As highlighted above, since the SONET trafficfollows the alignment property, the slot t_(i) is such that (t_(i) mods_(i))≡1. The defragmentation problem is thus defined as finding a newmap Ψ′={d_(i)→t_(i)′} that satisfies the alignment property and that“optimally packs” the link.

A link is defined as being “optimally packed” if demands occupying thelink are arranged such that the link can be filled completely tocapacity with the fewest number of new circuit demands. The optimalpacking ensures that it will satisfy any new demand set that can besatisfied by any other arrangement of current demands.

The goal of any defragmentation operation is to create a link that isoptimally packed. A link can, however, be optimally packed multipleways. The traditional approach is to perform a PW operation as shown inFIG. 1. As an aside, though PW is itself straightforward, the order inwhich bridge-n-roll circuit moves are performed in a PW operation isnot.

FIG. 3 illustrates the slots on a single link to further demonstratethis. FIG. 3 shows an OC-48 link 310 a that is fragmented by existingdemands. The PW operation moves all circuits to one end as shown in anOC-48 link 310 b. It is straightforward to see that the OC-48 link 310 bis optimally packed since it contains no “holes.” It should also beapparent that achieving this optimal packing structure requires sevencircuit moves (as stated above, however, the order in which the circuitmoves occur may not be as apparent).

Consider now the OC-48 link 310 c in FIG. 3. Note that it is alsooptimally packed. This can be verified by noting that the same set ofdemands that fill the PW frame to capacity (one STS-12c slot, two STS-3cslots and two STS-1 slots) also do the same in this case. Moreimportantly, only five circuit moves are required for the transitionfrom the original OC-48 link 310 a, two fewer than the OC-48 link 310 bthan was subjected to a mere PW operation. Consequently, given that boththe OC-48 links 310 b, 310 c exhibit optimal packing, the OC-48 link 310c is preferable, since it incurs a lower transition cost. Thus, a PWoperation is not the preferable solution in this environment.

The example in FIG. 3 demonstrated that a link can be optimally packedmultiple ways. The goal of defragmentation is to reach optimality whileincurring the smallest number of bridge-n-roll operations. It will benoted, however, that among all the ways to optimally pack the existingdemands in a link, determining the one among them that requires fewestcircuits to move appears to be theoretically hard (intractable, andperhaps NP-hard).

To work around the intractability of the problem, the disclosedalgorithm uses a heuristic to determine the optimal packing structurethat incurs a low cost of movement. After presenting the algorithm anddescribing its operation, performance numbers will be presented thatshow that the heuristic is indeed accurate and competitive with theoptimal.

Based on the optimality definition, one needs to know the fewest set ofdemands that would completely fill up a link if it were optimallypacked. Stated differently, the defragmentation algorithm needs tocreate “optimal defragmentation” (ODF) of free space mirroring each ofthese demands in the set. Thus, the OC-48 link 310 a in FIG. 3 wouldhave an optimally defragmented configuration if it had an empty STS-12cslot, two empty STS-3c slots and two empty STS-1 slots after thedefragmentation process.

The disclosed algorithm works in three phases:

(I) optimal defragmentation calculation, in which the optimaldefragmentation of the link(s) is calculated.

(II) free slot selection in which, for each granularity, minimum costcandidate slots of that granularity that should be freed are found. Thecost of freeing a candidate slot may be defined based on the number ofcircuits that need to be moved or the granularity of circuits that needto be moved.

(III) circuit movement, in which are determined the slots into which thecircuits identified in phase II can be moved without affectingearlier-cleared slots.

Turning now to FIG. 4, illustrated is a flow diagram of a method,generally designated 400, of increasing provisionable bandwidth in amultiplexed communication link carried out according to the principlesof the present invention. Each of the three above-mentioned phases willnow be described in greater detail with reference to FIG. 4.

Phase I: ODF Calculation

To calculate the ODF (a step 410 of the method 400), the optimal breakupof free space should be computed. To determine the number of slots thatcan be freed for the lowest granularity, the number of slots of thatgranularity that are present in the system and how many of them arebeing used should be determined. For any higher granularity, the slotsthat can be freed depend on how many slots of that granularity arepresent in that system and how many slots can be created. This is donein the illustrated embodiment by combining the free slots of lowergranularity (since each higher granularity is a multiple of the lowergranularity).

For a single link or same-sized multiple links with no nailed downcircuits, the ODF computation may be simplified. First calculate thetotal number of free slots, S. S is divided by the first SONETgranularity on the link(s) that is less than S to yield the number ofcircuits of that granularity in the ODF set. This operation is repeatedwith the remainder on the next lower granularity and so on. Thus, with20 free slots in the link 310 a of FIG. 3, the ODF set was 1 STS-12c, 2STS-3c (from the remainder of 8) and 2 STS-1 (from the remainder of 2).

If some circuits are nailed down and cannot be moved by thedefragmentation operation or if links are of different sizes, the ODFcannot be calculated by merely dividing the total number of free slotsby the granularity. In case of nailed down circuits, the availability offree slots by itself is not enough to create a free slot of highergranularity due to the inability to move these circuits. In case oflinks of different sizes, the availability of free slots by itself isnot enough to create higher granularity slots, as some links may notprovide a cross-connect at the higher granularity. A general formula forthe ODF can now be derived.

Let D_(g) equal the number of demands of SONET granularity (XCRate) g inall the links. Let F_(x,y) equal XCRate x/XCRate y. Let P_(g) equal thetotal number of possible slots of XCRate g in all of the links. Finally,let U_(g) equal the total number of slots of XCRate g that are taken bythe demands of XCRate g and higher. Thus U_(g)=D_(g)+ΣU_(g)★F_(x,g)∀x,where XCRate x>XCRate g.

Let A_(g) equal the total number of slots of XCRate g that can be freedacross all links. Then, A_(g)=min(A_(g−1)/F_(g,g−1),P_(g)−U_(g)). UsingA_(g), C_(g) (the contribution of XCRate g on the ODF) is computed asfollows:C _(g) =A _(g) −A _(g+1) ★F _(g+1,g).

This formula then provides the ODF.

Phase II: Free Slot Selection

Given that the ODF set is available from phase I, specific slots can nowbe chosen to be “cleared” to achieve the required set of contiguous freeslots (a step 420 of the method 400). If intractability is desired to beavoided, this may be achieved by a greedy heuristic that chooses at eachSONET granularity level, the requisite number of slots of thatgranularity whose clearing would require the fewest circuits to move.The slots that contain nailed down circuits are not considered forclearing.

Multiple links are combined into one “megalink” by linearlyconcatenating them. Appropriate slots are marked with highest circuitrate supported based on granularity of the link and adjacency of naileddown circuits. The algorithm operates on a link that may be a “megalink”or a single link. Pseudocode that reflects this algorithm is presentedin FIG. 5A.

At the end of phase II, the list of demands L that need to be moved tocreate the optimal breakup of free space is determined. In addition, foreach demand in L, a set of slots is identified (determined by a bit maskcorresponding to its granularity level) to which this demand may not bemoved into. This is to ensure that the slots cleared for a highergranularity are not fragmented again by moving a demand of a lowergranularity into those slots. Note that in step 8 of FIG. 5A, it isapparent from the way mask BM_(j) is obtained, that all demands to bemoved to free up space for granularity g_(j) will be of granularitylower than g_(j). This dependency in movement is accounted for by addingall these lower granularity demands to be moved to the ODF value. Therecursion breaks at lowest granularity where a demand can be movedanywhere as long as one slot is free.

Phase III: Circuit Movement

Now it comes time to move circuits (a step 430 of the method 400). Thelist L identifies all demands that need to be moved to create therequired set of contiguous free slots to account for the ODF set. Thebit map corresponding to each granularity also highlights the slotsavailable to move each of these demands into. Starting with the lowestgranularity (STS-1) and working up, free slots provably exist toaccommodate the demands to be moved. In other words, sufficient freesingle slots exist to move the STS-1 demands in L. The movement of theseSTS-1s will in turn create sufficient three-slot spaces for STS-3demands and so on. Pseudocode that reflects this algorithm is presentedin FIG. 5B.

The disclosed algorithm results in optimal packing of demands, but doesnot guarantee minimum number of movements. For this reason, analternative algorithm that achieves optimal packing in optimal number ofmoves will now be disclosed. Due to the perceived inherent complexity ofthe problem, this scheme involves an exhaustive search, but its runningtime can be curtailed by employing effective pruning mechanisms.

The algorithm takes the solution of the above-disclosed three-phasealgorithm as the currently-known best solution and tries to improve upon it by exhaustively searching the solution space. The algorithm issimilar to phase II, pseudocode for which is set forth in FIG. 5A.Except now instead of looking greedily for the local best as in step 5thereof, it explores all possible combinations at each granularity levelby recursively invoking the same function. Any exploration that incursmore moves then the current best, is abandoned. The movement step issame as phase III of the disclosed algorithm (FIG. 5B).

Turning now to FIG. 6, illustrated is a block diagram of a system forincreasing provisionable bandwidth in a multiplexed communication linkconstructed according to the principles of the present invention.

The system, generally designated 600, includes an optimaldefragmentation calculator 610. The optimal defragmentation calculator610 calculates an optimal defragmentation of the communication link thatis to be defragmented. The optimal defragmentation contains at least onegranularity (STS-1, STS-3c, STS-12c, etc.). In the specificcommunication link, the at least one granularity is a plurality ofgranularities related to one another as integer multiples (obeying thecontainment property described above).

The system 600 also includes a candidate slot selector 620. Thecandidate slot selector 620 is associated with the optimaldefragmentation calculator 610 and finds minimum cost candidate slotsfor the at least one granularity.

The system 600 further includes a circuit mover 630. The circuit mover630 is associated with the candidate slot selector 620 and identifiestarget slots into which circuits occupying the candidate slotsidentified by the candidate slot selector 620 can be moved.

In the system 600 of FIG. 6, the candidate slot selector 620 isconfigurable to determine the cost of a candidate slot based on eitherof (1) a number of circuits to be moved or (2) a granularity of circuitsto be moved.

The candidate slot selector 620 respects the optional constraintsdescribed above. More specifically, the candidate slot selector 620disqualifies slots containing nailed-down circuits from being thecandidate slots and further finds only aligned slots as candidate slotsfor said at least on granularity (obeying the alignment property).

Turning now to FIG. 7, illustrated is a block diagram of an NMS thatincorporates the method of FIG. 4 or the system of FIG. 6. The NMS,generally designated 700, includes a port 710 couplable to an opticalcommunication link 720. (The optical communication link 720 may be aSONET or SDH link and is environmental to the NMS 700. The NMS 700further includes a processor 730 coupled to the port 710. The processor730 allocates time slots of the optical communication link 720 todiscrete communication channels to effect a multiplexing of the channelsover the optical communication link 720. The processor 730 executes asequence of software instructions (not referenced) that carries out themethod 400 of FIG. 4 or embodies the system 600 of FIG. 6. Those skilledin the pertinent art understand, however, that the NMS 700 may carry outor embody the method 400 or the system 600 with dedicated hardware inlieu of or in addition to the processor 730 and sequence ofinstructions.

Having discussed ways to embody the disclosed algorithm, its performancewill now be evaluated in terms of the number of moves required to createa defragmented frame. Recall that optimizing the number of moves isintractable and perhaps NP-hard. While no known polynomial timealgorithm exists to determine the optimal number of moves, the optimalnumber of moves may be found by an exhaustive search procedure as givenin previous section.

Turning now to FIG. 8, illustrated are graphs 800, 810 that show howwell the heuristic performs against the optimal algorithm for STS-48(the graph 800) and STS-192 (the graph 810) links. The tests are basedon 2000 randomly generated frames. The graphs 800, 810 show that theheuristic's performance is extremely good, matching the optimal numberof moves in 95% and 70% of the cases for the OC-48 and OC-192 linksrespectively. For the remainder of the cases, the heuristic is only afew moves away from optimal algorithm, making the heuristic an optimaldefragmentation algorithm with near-optimal cost.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

1. A system for increasing provisionable bandwidth in a multiplexedcommunication link, comprising: a defragmentation calculator thatcalculates a defragmentation of said communication link, saiddefragmentation containing at least one granularity; a candidate slotselector, associated with said defragmentation calculator, that findslower cost candidate slots for said at least one granularity; and acircuit mover, associated with said candidate slot selector, thatidentifies target slots into which circuits occupying said candidateslots are moved.
 2. The system as recited in claim 1 wherein saidcandidate slot selector determines a cost of a candidate slot based onone of: a number of circuits to be moved; and a granularity of circuitsto be moved.
 3. The system as recited in claim 1 wherein said candidateslot selector disqualifies slots containing nailed-down circuits frombeing said candidate slots.
 4. The system as recited in claim 1 whereinsaid candidate slot selector finds only aligned slots as candidate slotsfor said at least on granularity.
 5. The system as recited in claim 1wherein said at least one granularity is a plurality of granularitiesrelated to one another as integer multiples.
 6. The system as recited inclaim 1 wherein said multiplexed communication link is time-divisionmultiplexed and said slots are time slots.
 7. The system as recited inclaim 1 wherein said multiplexed communication link is one of: a SONETlink; and an SDH link.
 8. A method of increasing provisionable bandwidthin a multiplexed communication link, comprising: calculating adefragmentation of said communication link, said defragmentationcontaining at least one granularity; finding reduced cost candidateslots for said at least one granularity; and identifying target slotsinto which circuits occupying said candidate slots are moved.
 9. Themethod as recited in claim 8 wherein finding comprises determining acost of a candidate slot based on one of: a number of circuits to bemoved; and a granularity of circuits to be moved.
 10. The method asrecited in claim 8 wherein said finding comprises disqualifying slotscontaining nailed-down circuits from being said candidate slots.
 11. Themethod as recited in claim 8 wherein said finding comprises finding onlyaligned slots as candidate slots for said at least on granularity. 12.The method as recited in claim 8 wherein said at least one granularityis a plurality of granularities related to one another as integermultiples.
 13. The method as recited in claim 8 wherein said multiplexedcommunication link is time-division multiplexed and said slots are timeslots.
 14. The method as recited in claim 8 wherein said multiplexedcommunication link is one of: a SONET link; and an SDH link.
 15. Anetwork management system, comprising: a port couplable to an opticalcommunication link; a processor, coupled to said port, for allocatingtime slots of said optical communication link to discrete communicationchannels; and a system for increasing provisionable bandwidth in saidoptical communication link, including: a defragmentation calculator thatcalculates a defragmentation of said optical communication link, saiddefragmentation containing at least one granularity, a candidate slotselector, associated with said defragmentation calculator, that findsreduced cost candidate time slots for said at least one granularity, anda circuit mover, associated with said candidate slot selector, thatidentifies target time slots into which circuits occupying saidcandidate time slots are moved.
 16. The network management system asrecited in claim 15 wherein said candidate slot selector determines acost of a candidate slot based on one of: a number of circuits to bemoved; and a granularity of circuits to be moved.
 17. The networkmanagement system as recited in claim 15 wherein said candidate slotselector disqualifies time slots containing nailed-down circuits frombeing said candidate time slots.
 18. The network management system asrecited in claim 15 wherein said candidate slot selector finds onlyaligned time slots as candidate time slots for said at least ongranularity.
 19. The network management system as recited in claim 15wherein said at least one granularity is a plurality of granularitiesrelated to one another as integer multiples.
 20. The network managementsystem as recited in claim 15 wherein said multiplexed communicationlink is one of: a SONET link; and an SDH link.